Some watercraft are powered by one or more water jet propulsion systems. The water jet propulsion system typically consists of a water jet pump which pressurizes water to create thrust. The water is first scooped from under the hull and flows in a water intake ramp to the water jet propulsion system, which expels it through a venturi as a water jet rearwardly of the watercraft. The water jet propulsion system is driven by an engine disposed inside the watercraft.
The water jet pump includes an impeller connected to a driveshaft. The driveshaft, which is operated by the engine, forces the impeller to rotate at high speeds when the watercraft is in operation. The impeller and a portion of the driveshaft are disposed in the water intake ramp surrounded by water, while a remainder of the driveshaft is disposed inside the hull, and connects to the engine. In those watercraft where the driveshaft is connected to the crankshaft by bulky couplings, the driveshaft is typically supported by one or more bearings in the vicinity of a front wall of the water intake ramp formed within the hull. A portion of the hull within the water intake ramp has an aperture equipped with a through-hull fitting through which the driveshaft extends. Typically, the aperture is of larger diameter than the driveshaft, and pressurized water from the intake ramp may flow through the aperture. Also, engine oil within the engine housing and used for lubricating parts of the engine such as the crankshaft bearings alongside the crankshaft through the crankcase cover may, in some cases, leak outside the engine and enter the inside of the hull.
To prevent water and/or engine oil to enter the inside of the hull, the driveshaft may be equipped with one or more sealing units (also called stuffing boxes or stern tubes). The sealing units consist of, for example, one or more ring seals. When sealing units are used to prevent both engine oil and water from flowing, the sealing units are two units separate from each other. One sealing unit is disposed at a portion of the driveshaft by the aperture of the hull receiving the driveshaft for preventing water to flow inside the hull, and another sealing unit is disposed at another portion of the engine housing or crankcase cover to prevent engine oil to flow inside the hull. A remainder of the driveshaft is deprived of sealing units, as it is sometimes equipped with two half jaw type couplings with an elastomeric element in between, which may be too bulky to be incorporated into any of the sealing.
Having two separate sealing units may become bulky, expensive and require space that creates weight distribution challenges within a crowded hull volume. Some sealing units are particularly complex in design and can become difficult to assemble.
Therefore, there is a need for a watercraft and a propulsion system for a watercraft with a sealing unit that would be simple, inexpensive, compact, and that would provide a more reliable sealing as currently available.